Galaxy Formation on a Benchtop

By Matthew Francis, Ars Technica

For a variety of obvious reasons, it’s impossible to reproduce the exact environment in which galaxies form. The lack of direct experimental tests for the models astrophysicists use creates a disconnect between what astronomers observe and theoretical work. However, that barrier is being broken down by a combination of high-powered lasers and a new understanding of how lab-scale experiments can be related to vastly larger systems such as galaxies.

[partner id=”arstechnica” align=”right”]Researchers at the Laboratoire pour l’Utilisation de Lasers Intenses (LULI), along with colleagues at various universities, have successfully simulated the magnetic fields that form in early galaxies. Naively, there seems to be no correspondence between the experiment and the real astrophysical system. The lab set-up is very small, works on a very short time frame, and uses carbon rods and lasers; the real environment for galaxy formation is clouds of gas and dark matter, and the time-scale is hundreds of millions of years. Nevertheless, a magnetic field strength (along with other effects) has been observed in the lab that corresponds to that experienced by early protogalaxies.

In galaxy formation models, a gravitational nucleus is formed out of cold dark matter. Ordinary matter in the form of gas collects around the nucleus and, as it collapses, it heats up. The relatively rapid gravitational collapse sends shock waves through the gas, blowing some of it away from the protogalaxy, but driving star formation in the process. (A shock wave is a wave that travels faster than the speed of sound in a material, as with a sonic boom.)

Because this formation is happening on a large physical scale (since galaxies are on the scale of tens or hundreds of thousands of light-years across), some parts of the protogalaxy will be more dense than others, which means the shock waves will be unevenly distributed. The ionizing effect of the shocks strips atoms of their electrons; the accelerating charged particles then produce magnetic fields. This process is known as the Biermann battery.

Numerical simulations and comparison with observational data bear out the Biermann battery model, but how to test it in the laboratory? The solution is to use a series of physical analogies. For clouds of gas, the researchers substitute a carbon rod immersed in low pressure helium. Instead of gravitational collapse to drive the shock waves, they use intense short bursts of laser light.

The rod is 0.5 millimeters in diameter, and it is subjected to either one or two laser pulses, each of which are about 0.4 millimeters wide and that last about 1.5 nanoseconds. The combination of a relatively wide laser beam and very high energy sends shock waves out from the carbon rod into the gas. The both the strength and direction of the magnetic field can be measured in three dimensions using induction coils.

When the laser strikes the carbon rod, the rod expands dramatically and ionizes the gas, sending hot electrons in a wave outward. The shock wave is not perfectly spherical, which agrees with galaxy formation scenarios. That’s quite important, as perfectly spherical shock waves do not produce magnetic fields, according to standard models.The magnetic induction coils, placed at two different distances from the blast center, were able to measure the evolution of the wave shape as it dissipates.

The magnetic field is produced directly at the wave front, so it is strongest when the shock passes the detector, and weakens after that. (The researchers also noted a second peak in the magnetic field, when the material blasted off the carbon rod reaches the detector, which has no analog in astrophysical systems.) The entire experiment occurs over a period of a few nanoseconds, but high-resolution instruments are able to track the shock waves and confirm their correlation with the magnetic field peaks.

The researchers looked at two different gas pressures inside the helium, and compared both to the results generated without helium. The model predicts that the helium is the source of the electrons, which themselves produce the magnetic fields; as expected, the experiment with no helium gas did not produce the strong magnetic fields. The lower-pressure trials generated slightly higher magnetic fields, again to be expected since higher pressure means higher density of gas, which slows the shock wave formation.

Relating the experimental results back to astronomy involves dramatic rescaling. The time-frame goes from a few nanoseconds in the lab to approximately 700 million years for gravitational collapse, and the relatively high magnetic field strength in the lab (from the large number of electrons in a small space) subsequently becomes much smaller. By using standard scaling formulas, the magnetic fields observed correspond to each other—a dramatic confirmation that non-spherical shock waves during galaxy formation are indeed the source of the galactic magnetic fields we’ve observed.